A simple electromagneticacoustic wavetransducer (EMAT) based on a wire coil was used to excite the resonant modes of a stainless steel spherical shell without direct mechanical contact. The coupling produced by this EMAT was examined first for shells in air and then for shells immersed in water to examine the effects of fluid loading on the shell’s spectrum. It was found that the torsional modes were excited using this method and these modes radiated sound into the surrounding water contrary to expectations. This excitation is shown to depend on the presence of a permanent magnetization in the shell itself or on the presence of a static external field applied at right angles to the axis of the coil. Possible mechanisms for the excitation and the acoustic radiation of the torsional modes are considered. The excitation of quasi-flexural shell modes is also discussed for shells in air and in water. The shell responds at the oscillation frequency of the applied field and at twice the frequency. Some potential applications of this method of measuring modes are noted.

Measurements and predictions of the response of a variety of plain and coupled plates when excited by a point source have been reported. Tests on plain plates showed that a diffuse field was generated within the first 30 wave transits across the plate. In plates coupled by a thin ligament, around 50 wave transits across one of the substructures was required for a diffuse field to be established in the whole system. This is equivalent to about 25 transits across the whole plate so the time required to set up a diffuse field was not significantly affected, even when the ligament width was only 2.5% of the total plate width. Tests on plates bolted or adhesively bonded together showed that in both cases, although a diffuse field was established in each of the substructures, the damping was too great for a diffuse field to be set up in the whole structure. Tests on stepped plates have shown that the measured surface amplitudes in the thinner sections tend to be much larger than those in the thick sections, but that the energy in the thinner sections is only slightly larger than that in a thick section of similar plan area. The field is not diffuse in the sense that the amplitude or energy density is the same throughout the coupled structure, but the field is diffuse in each substructure.

This paper concerns the development of a method adapted for constructing reduced models in the medium-frequency range to a general three-dimensional dissipative structure consisting of an anisotropic, inhomogeneous, viscoelastic bounded medium coupled with an internal acoustic cavity. The reduced models are obtained using the Ritz–Galerkin method for which the projection subspace corresponds to the dominant eigensubspace of the energy operator of the structure in the medium-frequency band of analysis. Two fundamental cases are considered: (1) both the structure and the internal acoustic cavity have a medium-frequency behavior in the frequency band of analysis; (2) the structure has a medium-frequency behavior in the frequency band of analysis while the internal acoustic cavity has a low-frequency behavior.

The paper investigates the hypothetical assumption of neglecting transverse normal stress in vibration analysis for cantilevered thick plates and rectangular parallelepiped. The analysis solves the three-dimensional elasticity energy functional including, as well as excluding, transverse normal stress and obtains free vibration solutions for a cantilevered parallelepiped. Although it is widely accepted, the omission of transverse normal stress is well justified in Kirchhoff–Love thin-plate theory and higher-order thick-plate models; the transverse normal stress effects and thickness extent to which the thick-plate models apply as the thickness increases are practically unknown. The inconsistency of assuming constant transverse normal displacement through thickness for thick-plate models is also addressed. The paper concludes that for a rectangular parallelepiped with thickness exceeding a certain limit, there is considerable discrepancy if transverse normal stress is neglected.

This paper presents a technique to assess the impact on model fidelity introduced through discretization of attachments on harmonically forced fluid-loaded structural acousticmodels. While fluid loading is included, it is not a requirement or restriction to the methods presented. The perspective taken is one of knowledge of a reference state, with a desire to determine the impact on the total radiated acoustic power due to perturbations in the reference state. Such perturbations change the predicted resonance frequencies of a structure under consideration and, hence, change the predicted response amplitudes. The method uses a single degree of freedom response model in the local region of each fluid-loaded resonance, coupled with eigenvalue sensitivities, to estimate the perturbation impact. The sensitivity of the eigenvalues to changes in model detail is derived based on variations in the spatial representation of attached features (e.g., point versus distributed attachments). Elements of the analysis method are not necessarily restricted to modelperturbations nor acoustic power, rather they may be used to assess the perturbation of any quadratic response quantity of interest due to changes in resonance frequency. The analysis reveals that the bandwidth of response perturbation increases with increasing resonance frequency. For frequencies within ±5% of a resonance frequency, the amount of damping in the system determines and limits the magnitude of the response perturbation. The perturbation outside the range of ±5% of the resonance frequency is relatively insensitive to damping. The SDOF analysis method is limited by its assumption of constant modal forcing between the reference and perturbed states.

When designing an active control system to globally control the far-fieldsound radiation from a vibrating surface, a challenging problem is to properly define the near-field acoustic sensing strategy and the type of cost function to be minimized by the controller. The strategy of sensing and minimizing the near-field active intensity at discrete locations in the active control of free field radiation from a vibrating plate is investigated in this paper. The optimal minimization of the sum of the near-field, normal active sound intensities at the error sensor locations using acoustic controlsources is derived for this problem, and the results obtained are compared to the minimization of the sum of the near-field squared pressures. Some of the difficulties associated with sound intensity minimization are pointed out.

A band-limited method of selecting actuators and sensors for structural acoustic control is reviewed, and experimental results are presented to demonstrate the approach. The selection methodology is based upon the decomposition of the Hankel singular values of a system model in terms of individual sensor and actuator configurations for lightly damped structures. The technique selects sensor and actuator combinations which couple well to structural modes that radiate efficiently. However, it rejects sensor and actuator combinations which couple well to modes that are inefficient acoustic radiators or are outside of the desired bandwidth of control. Selecting transducer combinations which filter modes outside of the desired bandwidth serves to minimize the potential for spillover and instability associated with unmodeled or poorly modeled dynamics. The approach is computationally efficient since it is based upon open-loop dynamics and does not require iterative nonlinear optimization.